Manganese oxide based catalyst and electrode for alkaline electrochemical system and method of its production

ABSTRACT

A catalyst, active layer, and cathode for metal air and other air-assisted cells and methods of producing these are disclosed. The cathode produced comprises a substantially unoxidized carbon support with a manganese or other oxide catalyst. The support maintains its inherent water repellency, conductivity and active sites. The cathode is therefore capable of sustaining significantly high currents for prolonged duration, at higher operating voltages, enabling the extension of metal air technology for higher power devices.

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims the benefit of U.S. patent applicationSer. No. 10/066,938, filed Feb. 4, 2002, the text of which isincorporated herein by reference.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

[0002] Not applicable.

BACKGROUND OF THE INVENTION

[0003] The present invention relates generally to the fabrication ofcarbon-based air cathodes loaded with manganese or other oxides formetal air cells, air-assisted alkaline cells and fuel cells, and inparticular, relates to the preparation of manganese based oxides forsuch cathodes.

[0004] Traditional metal-air batteries are usually disk-like inappearance and are therefore referred to commonly as button or coincells. Other configurations, including cylindrical metal air cells, areknown and are applicable to the invention described herein. Metal aircells are disclosed in several patents including U.S. Pat. No. 5,721,065issued Feb. 24, 1998, assigned to Rayovac Corporation, and entitled “LowMercury, High Discharge Rate Electrochemical Cell,” and U.S. Pat. No.6,197,445 issued Mar. 6, 2001, assigned to Rayovac Corporation, andentitled “Air Depolarized Electrochemical Cells,” the disclosures ofwhich are both incorporated by reference herein as if set forth in theirentirety.

[0005] Of the potential metal-air battery candidates, zinc-air hasreceived the most attention because zinc is the most electropositivemetal, and is relatively stable in aqueous and alkaline electrolyteswithout significant corrosion. In a zinc-air battery, the anode containszinc and, during discharge, oxygen from the ambient air and water fromthe electrolyte are converted at the cathode to hydroxide, the hydroxideoxidizes the zinc at the anode, and water and electrons are released toprovide electrical energy.

[0006] In metal-air batteries, a reactive metallic anode iselectrochemically coupled to a carbon-based air cathode through asuitable alkaline electrolyte. As is well known in the art, the aircathode is typically a sheet-like member having a surface exposed to theatmosphere (air) and a surface exposed to an aqueous electrolyte of thecell. During operation, oxygen from the air dissociates and is reducedat the cathode, while metal of the anode is oxidized, thereby providinga usable electric current flow through the external circuit between theanode and the cathode. Metal air cells achieve very high energydensities, as the air cathode is very compact yet has essentiallyunlimited capacity. Because most of the volume is available for theanode active material, a metal-air cell can provide more watt-hours ofelectromotive force than a typical “two-electrode cell” of similar size,mass and anode composition that contains both anode- and cathode-activematerials in approximately equal amounts inside the cell structure.

[0007] A stable gas/liquid/solid interface is important to effectivedischarge of metal air cells. Conventionally, the air cathode includes asubstrate-supported active layer and an air diffusion layer. The activelayer comprises a mixture of a carbon support, one or more fine particlecatalysts on the support, and a polymeric binder/waterproofing agentsuch as polytetrafluorethylene (PTFE). The active layer is adhered orlaminated to a metallic current-collecting substrate. The substrate istypically a cross-bonded screen having nickel metal strands woventherein, or a fine-mesh expanded metal screen. The air diffusion layerusually includes one or more pure hydrophobic membrane layers laminatedto the air side of the active layer. Some metal air cells for highcurrent drain applications employ a dual layer which includes a passive,hydrophobic barrier layer between the active layer and the air diffusionlayer. This additional layer makes processing complex and expensivebecause it has to be fabricated separately then bonded to the activelayer.

[0008] It is generally understood that a two-step oxygen reductionprocess occurs in metal air cells. The process requires diffusion anddissolution of oxygen gas and electrochemical reduction. A two-electronreduction at the carbon support surface produces peroxide ions. Theperoxide ions are subsequently reduced in a catalytic step facilitatedby the oxide catalyst such.

[0009] The entire composite electrode structure must have highelectronic conductivity to ensure effective collection of the currentand to reduce ohmic resistance. Without this, an undesirable voltagedrop results. In addition to ohmic voltage drop, kinetic (reaction ratedependent) and mass transfer polarization can also reduce the cellvoltage. For example, without sufficient available oxygen reductionsites or catalyst, a voltage drop due to kinetic limitations can occur.Insufficient catalyst alone can slow the catalytic step and causeperoxide accumulation, leading to a lower voltage. At high currentdensities, the cathode is generally under mass transfer control, meaningthat mass transport becomes the rate determining step. Hence poor masstransport of reactants (e.g. oxygen) or products, which can be caused bylow cathode porosity or by excessive wetting of the electrode,significantly increases polarization and reduces operating voltage.Generally, as the current demand increases during operation, it isbelieved that the reaction front moves outward toward the air side ofthe cathode, and more of the cathode surface area participates in thereaction. The liquid electrolyte can film over or flood the cathodesurface, thereby blocking air access and reducing the active (available)three-phase interfacial area for reaction. It can eventually breakthrough to the air side of a structurally deficient electrode and puddlebetween the cathode and the laminated hydrophobic barrier layer. Acommonly used term for this condition is “cathode flooding.” The netresult of all these phenomena is that the electrode cannot sustain thecurrent density resulting in premature battery failure. Wetting-throughof the cathode by electrolyte is further detrimental since theelectrolyte is corrosive and leakage can cause damage to expensivecomponents.

[0010] If the carbon support and the entire electrode structure becometoo hydrophilic for any reason, wetting through of the cathode andperformance degradation as noted above cause premature failure.

[0011] In a conventional metal air cathode, one can compensate for aloss in conductivity by adding conducting carbon black. One can alsovary, within limits, the amount of the hydrophobic binder in the activelayer and the processing conditions to maintain a cathode having ahydrophobic character. However, an increase in the amount of the bindercan undesirably reduce the electrode porosity and the number of carbonsites available for reaction. This hinders effective mass transport,which is critical at high current densities. Furthermore, using morebinder increases the material cost of the electrodes.

[0012] As new high power, high current density devices raise performanceexpectations, the requirements for sustaining oxygen reduction over thelife of the battery are becoming more demanding.

[0013] It is therefore a goal of air cathode design to increase oxygenreduction and reduce polarization from all sources. For example,attention has already been directed to the catalyst support, thecatalyst particles, and on the cathode layer structures employed. Thecarbon support must have sufficient sites for the oxygen reductionreaction. This depends strongly on the type of carbon, as well as itssurface area and surface functionalities. These attributes can dependupon the starting materials used to produce the carbon support and themethod of its manufacture.

[0014] Also, inexpensive highly active, fine organic and inorganiccatalyst particles should be well distributed throughout the carbonsupport to ensure rapid and effective consumption of all the peroxideproduced to ensure high operating voltage. However, the choice ofmaterials is limited because the catalyst must not only have a highoxygen-reducing activity, but must also withstand the corrosiveenvironment of an electrochemical cell. The availability, cost andenvironmental or toxicological effects of the materials also have abearing on the suitable choice for large-scale use of the material inpractical systems for consumer applications.

[0015] Manganese oxides are known to be suitable catalysts forcarbon-based air cathodes, and various methods are known for producingoxide catalyst on the carbon support. Most methods react the carbon witha strong oxidizing agent such as potassium permanganate (KMnO₄) orsilver permanganate (AgMnO₄). The KMnO₄ is reduced to MnO₂, while thecarbon is oxidized and eventually produces K₂CO₃. For example, U.S. Pat.No. 4,433,035 and U.S. Pat. No. 5,378,562 both disclose reducingpotassium permanganate with either carbon black or activated carbon toproduce carbon based air cathodes loaded with manganese oxide. U.S. Pat.No. 3,948,684 teaches using KMnO₄ and/or heat to deposit MnO₂ catalyston carbon, and also suggests that MnO₂ production is facilitated byusing H₂O₂ with the KMnO₄. Both H₂O₂ and KMnO₄ are strong oxidizingagents that can rapidly oxidize the carbon surface. U.S. Pat. No.3,948,684 taught that the electrodes performed better when Mn(NO₃)₂ wasused. U.S. Pat. No. 4,433,035 describes using KMnO₄ as an oxidizingagent on carbon black while adding “uncatalyzed” carbon black,presumably to compensate for the loss in conductivity.

[0016] U.S. Pat. No. 5,378,562 describes using KMnO₄ on carbon the roomtemperature to produce Mn⁺². FIGS. 5 & 6 in their patent show increasein impedance with increasing catalyst loading. While this methodeffectively produces well distributed, fine particle MnO₂, the cathodeswere developed exclusively for hearing aid battery development whichrequired no more than 10 mA/cm² at the time. At higher currents, theseelectrodes are prone to lower conductivity

[0017] Patent publication WO01/37358A2, entitled “Cathodes for Metal AirElectrochemical Cells” discloses an admixture of silver permanganate andcarbon black, wherein silver permanganate is reduced in situ by carbonblack to form a manganese oxide/silver catalyst mixture supported oncarbon, which is used as cathode for oxygen reduction. FIG. 5 in theirpatent shows that 10% MnO₂/C does not perform well compared to 5%MnO₂/C, suggesting that high catalyst loadings are detrimental.

[0018] Sol-gel processes have also been employed to produce MnO₂ formetal air cathodes. Sol-gel chemistry in aqueous solutions is based onthe hydrolysis and condensation of metal ions. Sols are colloidalsuspensions of the reaction product that are typically nano-sized. In a“true” sol, by virtue of the charge on the particles, the repulsiveforces between adjacent particles can keep them in suspension for longperiods of weeks to months. The particle size as well as theagglomeration or “aging” of particles depends on the concentration ofreactants and product in the liquid, the type of precursors used, therate of reaction, pH, etc.

[0019] According to Bach et. al., (J. Solid State Chem., 88,325-333,1990) to synthesize MnO₂ using sol-gel techniques, due to the lack ofstable Mn(IV) precursors, one can use redox reactions to obtain MnO₂rather than the typical acid-base type reactions in sol-gel synthesis.Hence soluble inorganic precursors like KMnO₄, NaMnO₄, LiMnO₄, AgMnO₄etc. can be reduced by appropriate organic or inorganic reducing agentsto produce a sol, suspension, slurry or gel depending on the materialand conditions used for the synthesis. The oxides produced from such lowtemperature techniques generally produce largely amorphous materials asdetermined by X ray diffraction analysis. The manganese oxides producedtypically have mixed valence states, although careful control of themolar ratio of reactants can ensure a mean oxidation state of +4 for theMn oxide. The sols can also be treated with acids to promote thedisproportionation of Mn³⁺ to Mn²⁺ and Mn⁴⁺. The Mn²⁺which is soluble,can be washed away, leaving largely Mn⁴⁺.

[0020] French patent 2,659,075, also by Bach et al., entitled “Sol-gelProcess for the Preparation of Manganese Oxide” discloses thefabrication of manganese oxide via the reduction of potassiumpermanganate solution with a carboxylic acid having four carbon atoms.This method produces a manganese (IV) oxide gel using fumaric acid asthe reducing agent. It is claimed that the four-carbon nature of thereducing agent yields a gel, in which the manganese oxide particles aresuspended. The objective of the patent is to produce crystalline MnO₂for reversible intercalation of Li ions in a rechargeable Li battery.Hence the MnO₂ gel is subjected to high temperature heat treatment(calcined) to produce the desired crystal structure and orientation.

[0021] U.S. Pat. No. 6,465,129 entitled ‘Lithium Batteries with NewManganese Oxide Materials as Lithium Intercalation Hosts” describes“sol-gel” technology and the importance of distinguishing betweenvarious methods, the different chemical and structural characteristicsof the synthesized material, and the end application of inventions. Itis incorporated herein by reference. The inventors describe nanoporous,amorphous MnO₂ with high lithium intercalating properties, which are notsubjected to high temperatures.

[0022] J. Electrochem. Soc., 143(5):1629 (May 1996) (Stadniychuk, et.Al.), incorporated by reference as if set forth herein in its entirety,surveys various methods for producing MnO₂. The paper describes theimportance of pH and concentration on sol-gel transition when usingfumaric acid as reducing agent. It also describes rather complex methodsof producing MnO₂ nanoparticles for use in thin film alkaline batterieshaving an electrode predominantly comprising MnO₂, where the MnO₂, isdirectly consumed in the reaction. In contrast, the MnO₂ of metal airbatteries behaves as a catalyst that facilitates a reaction but is notconsumed.

[0023] U.S. Pat. No. 6,444,609 entitled “Manganese-based OxygenReduction Catalyst, Metal-Air Electrode Including Said Catalyst andMethods for Making the Same Relates to a Sol-gel Process for Making aCatalyst for an Air Electrode.” The inventors disclose combining amanganese alkoxide of valence state +2 with alcohol under suitableconditions to produce a sol, converting the sol to a gel, mixing the gelwith carbon to produce a mixture, and then pyrolyzing the mixture at ahigh temperature to produce the MnO₂, which has valence state of +4, onthe carbon support.

[0024] An increasing demand for higher power cells has been created bynewer devices, such as hearing aids, particularly digital hearing aids.The desired increase in power demands that cells have an ability tooperate at higher voltages and at higher currents. Still higher powerdemands are seen in recent attempts to develop and produce largerbatteries in cylindrical or prismatic form, for consumer electronic aswell as military, applications. From processing and performancestandpoints, it is desirable to preserve the surface chemistry thatinfluences the physico-chemical properties such as wettability andelectrical properties of the support carbon materials.

BRIEF SUMMARY OF THE INVENTION

[0025] While, in general, it is known that it is important to maintainthe hydrophobicity of the metal air cathode, the prior art has notheretofore appreciated that, at high currents, cathodes can fail asoxidation at the carbon support surface promotes undesired surfacemicro-hydrophilicity. The inventors have determined the importance ofthree-dimensional hydrophobic/hydrophilic balance at the micro- andmacro levels in the electrode for sustaining high power and high currentdensity discharge. The inventors have further determined thatconventional processing methods cause physical or chemical oxidation ofthe carbon surface and that surface oxygen compounds increase thehydrophilicity of the carbon and make the carbon and the electrode morewettable. It is further believed that oxidation reduces electrochemicalactivity by consuming active sites that would otherwise participate inthe oxygen reduction reaction.

[0026] Accordingly, the present invention is summarized in that a carbonsupport substantially unoxidized during catalyst loading isadvantageously used in an air cathode of a metal air cell for highcurrent drain applications. The carbon support of the invention hasmicro-hydrophobic properties not seen in the prior art. It isconventional in the art for the carbon support to be provided on itssurface with an oxide catalyst that can comprise an oxide of manganese,silver or cobalt, or mixtures thereof, with a manganese oxide,particularly manganese dioxide, being the preferred oxide catalyst.References herein to a manganese compound, such as a permanganate or anoxide, are intended to encompass the other suitable catalysts orcatalyst precursors as well.

[0027] In a related aspect, a carbon support having a high level ofreactive sites and a low level of oxidation can be selected fromavailable carbon sources for mixing with existing oxide catalystparticles, and can be prepared as described herein such that the carbonis substantially unoxidized after the catalyst has been provided on thesupport. The carbon support can be activated carbon or conductive carbonblack of conventional size. A suitable activated carbon has a surfacearea of at least 200 m²/g, preferably greater than 700 m²/g. Activatedcarbons are most commonly obtained by steam or chemical activation ofpitch or coal based precursors, to produce extremely high porosityparticles with high adsorptive capacity for organic and inorganiccompounds. These properties are thought to make the materialsappropriate for oxygen reduction reactions, which can be furtherenhanced by incorporation of catalysts. For such carbons, the molassesnumber indicates internal porosity, and the iodine number, their surfacearea in m²/gm. Preferred activated carbons are PWA carbon (CalgonCorporation, Pittsburgh, Pa.)), which is bituminous coal derived, andhas a molasses number of 218 and iodine number of 900. Norit SX1G, aswell as other grades from Norit Americas Inc, Atlanta, Ga., are suitablepeat-based activated carbon with a molasses number of ˜310 and iodinenumber of 900. A suitable conductive carbon black has a surface area ofat least 1200 m²/g. It is preferred that a conductive carbon black havea surface area of at least 1200 m²/g. A preferred carbon black is BlackPearls 2000 or Vulcan XC 72 (Cabot Corporation, Billerica, Mass.).Another suitable carbon black is Ketjen Black (Akzo Nobel Corporation,Chicago).

[0028] The meaning of “substantially unoxidized” in this applicationrefers to further oxidation of the carbon support after the activationprocess used to produce the support materials. In other words, a goal ofthis invention is to avoid oxidizing the support while producing acatalyst-loaded support. Preferably, the support is not oxidized duringcathode loading in accord with the methods described herein, butoxidation to some extent can be tolerated, for example as much as about10-20%, or even more, oxidation can be acceptable depending upon theapplication. The extent of oxidation is best determined operationally byreference to the suitability of a cathode in a high rate application inthe manner shown in the accompanying Examples. The statement is notintended to suggest that the starting carbon is free ofoxygen-containing surface groups, but rather that the level of suchgroups is sufficiently low when loaded with catalyst so as not tosubstantially reduce the function of a cathode under high currentconditions as described herein (not more than 10% reduction relative tocathode fabricated similarly without regard to oxidation of the support,e.g., as in Example 1). The level of carbon support oxidation that isacceptable in the invention will vary with the activity of the startingcarbon material, and more particularly with its reactivity after cathodeformation in the oxygen reduction reaction described above. Accordingly,if the starting material has a large number of reactive sites, thecarbon can be partially oxidized without adverse impact upon cathodeactivity. In contrast, a relatively inactive starting material havingthe same proportion of oxidization can be rendered unusable in a cathodefor high rate applications.

[0029] In another aspect, the invention is further summarized in that anactive layer for a cathode for a metal air cell comprises a mixture of apolymeric hydrophobic binder and a carbon support of the inventionhaving supported on its surface the catalyst material. The active layermixture can be adhered or laminated to a metallic current-collectingsubstrate, and combined with one or more air diffusion layers in aconventional manner to form a cathode for a metal air cell. The cathodecan be incorporated into a metal air cell in a conventional manner. Acathode active layer of the invention typically comprises 70-80% carbonand 2-20% of the oxide catalyst, by weight, with the balance beingbinder.

[0030] In one aspect, a method for producing an active layer of theinvention begins with a method for producing an oxide catalystsuspension that will not oxidize a carbon support when the two are mixedtogether. In the method, an oxidizing agent, preferably a solublemanganese compound having a valence state higher than +4, is mixed withone or more suitable organic or inorganic reducing agents at atemperature in the range of 10° C. to 100° C. to produce a suspension ofoxide(s) containing particles ranging from sub-micron to several micronsin size, for example between 100 nanometers and 30 microns in size.

[0031] It should be appreciated that the present invention is intendedto include suspensions of various particle sizes, which may be producedby adjusting the starting materials and/or the reaction conditions in amanner known to the art. Substantially all of the primary particles arepreferably submicron size, but the primary particles can aggregate toform larger secondary clusters. While sub-micron sized primary oxideparticles are desirably employed in the method, the invention is notlimited to oxide catalyst having a specific particle size range. Forinstance, the Examples below demonstrate that oxide catalyst aggregateson the order of 20 microns in size can be used in a cathode having highcatalytic activity and hydrophobicity for sustained high current densityperformance. The oxide particle size distribution can be determinedusing a Coulter Particle Size Analyzer.

[0032] The oxidizing agent can be selected from a variety of compoundscontaining manganese of valence greater than +4. Permanganate salts arepreferred, for instance, lithium-, sodium-, potassium-, silver-,ammonium- or cobalt-based salts, or mixtures of the salts. A suitableorganic reducing agent can have one or more carbon atoms, and caninclude fumaric acid, citric acid, formic acid, or a salt of theseacids, as well as an alcohol, aldehyde, or the like that can be readilyoxidized. The reducing agent can also comprise one or more inorganiccompounds, such as a nitrate, chloride, sulfate, or perchlorate ofvarious cations, as well as hydrogen peroxide, and the like, whichreadily react with and reduce the oxidizing agent. The reducing agentcan further contain manganese in the +2 valence state (e.g., manganousnitrate, perchlorate,or sulfate) which is oxidized by the permanganateto a higher oxidation state (e.g., +4).

[0033] The mixing of the oxidizing and reducing agents can beaccomplished ex situ, under conditions that form particles having thedesired size. The catalyst particles are later mixed with a carbonsupport having the described attributes without exposing the support toan oxidizing agent. The particle suspension produced in an ex situmixing method is preferably a colloidal sol comprising the oxideparticles. The particle suspension is then transferred onto the carbonsupport (e.g., provided as a slurry, paste, or powder) under agitationto produce a substantially unoxidized carbon support loaded withcatalyst for further processing into a metal-air cathode. In a relatedaspect, the particles and the carbon support can have net oppositecharges that enable charge-induced attraction and adsorption of theparticles to the carbon surface. It is possible to adjust the depositionof the catalyst particles onto the support by incorporating surfactantsor other additives to modify the inherent or imparted charge on thesupport relative to the charge on the particles.

[0034] Alternatively, the oxide can be deposited in situ on the carbonsupport by mixing the oxidizing and reducing agents with the carbonsupport under conditions that favor a redox reaction between theoxidizing and reducing agents over the reaction between the oxidizingagent and the carbon. The rate of reaction between the reducing andoxidizing agents should be at least twice as fast, more preferably fiveto ten times as fast, as the rate of reaction between the oxidizingagent and the support. Under such conditions, the very fine particlesthat form in the redox reaction are immediately attracted to and attachto the carbon support, which can also act as a seed or nucleation site.A particle suspension produced in situ with the carbon support asdescribed is more intimately dispersed than would be the case forparticle produced ex situ and the carbon will still be substantiallyunoxidized.

[0035] It is an object of the invention to provide a carbon-supportedcatalyst for use in a cathode active layer suitable for use in a highperformance metal air cell to deliver high power and high currentdensity.

[0036] It is another object of the invention to provide the catalyst ata loading of between about 1% and 20%, preferably between about 5% and15%, oxide catalyst by weight in the cathode to ensure suitability foruse in a high performance metal air cell.

[0037] It is a feature of the invention that a cathode active layer ofthe invention comprises the carbon-supported catalyst of the invention.

[0038] It is another feature of the invention that the carbon supportcan be substantially unoxidized during the catalyst loading process.

[0039] It is also an advantage of the invention that the catalyst oxideparticles can, if desired, be produced in situ with the carbon support.

[0040] It is an advantage of the invention that the carbon supportmaintains adequate electrical conductivity and chemical reactivity forhigh current drain discharge applications.

[0041] It is another advantage that a cathode of the invention maintainscatalytic activity and conductivity, and retains a hydrophobic characterat both the macro (cathode) and micro (carbon support) levels, andthereby is sufficiently robust to sustain high current densities withminimal flooding.

[0042] It is yet an advantage of the invention that it does not requirea high temperature pyrolysis step to produce the Mn⁺⁴ oxide.

[0043] Still another advantage of the invention is that the catalyst canfunction in a single active layer.

[0044] It is yet another advantage of the present invention that readilyavailable, inexpensive compounds are employed in the making of thecarbon-supported catalyst of the invention.

[0045] It is a still further advantage of the invention that no gellingstep is required for the sols produced, thereby avoiding processingsteps and reducing costs.

[0046] A yet further advantage of the invention is that a wider range ofoxide catalyst loading is enabled, which is otherwise not possible dueto cathode deterioration effected by high catalyst loading in the priorart.

[0047] These and other aspects of the invention are not intended todefine the scope of the invention for which purpose claims are provided.In the following description, reference is made to the accompanyingdrawings which form a part hereof, and which there is shown by way ofillustration, and not limitation, preferred embodiments of theinvention. Such embodiments do not define the scope of the invention andreference must therefore be made to the claims for this purpose.

BRIEF DESCRIPTION OF THE DRAWINGS

[0048] Reference is hereby made to the following figures in which likereference numerals correspond to like elements throughout, and in which:

[0049]FIG. 1 is a schematic sectional side elevation view of a zinc-airbutton cell constructed in accordance with the invention;

[0050]FIG. 2 compares the effects of carbon oxidation on hydrophobicityof cathodes constructed in accordance with the preferred embodiments tothat of the prior art;

[0051]FIG. 3 compares the long-term stability and performance of acathode constructed in accordance with the preferred embodiments to thatof the prior art;

[0052]FIG. 4 is a graph illustrating the discharge profile of a zinc air13 size cell constructed in accordance with the preferred embodimentcompared to the prior art.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

[0053] In this application, high current density is defined as a currentdensity greater than about 50 mA/cm². A “high performance cell” operatesat a high current density and resists flooding and leakage.

[0054] While the preferred embodiments are described with reference to acathode for a button cell, it should be appreciated by a skilled artisanthat the present invention is equally applicable to the fabrication ofcathodes for other types of cells, including but not limited to othertypes of metal-air cells, fuel cells, or any other electrochemical cellsthat can benefit by having a carbon-supported oxide-based electrode.

[0055] Methods for forming a cathode from a catalyst-coated support anda binder are known and, except as noted, conventional cathode-formingmethods can be employed in this invention. Rather, the invention relatesto methods for producing a catalyst-coated support having the indicatedproperties and to the support, coated support, cathode comprising thecoated support, and cell comprising the cathode. In accordance with theinvention, methods for both ex situ and in situ production are describedbelow, after presenting the structure of a button cell constructed inaccordance with the invention.

[0056] Referring to FIG. 1, a metal-air cell, and in particular a buttoncell 10, is disposed in a battery cavity 12 of an appliance 14. Thecavity 12 is generally bounded by a bottom wall 16, a top wall 18, andside walls 20. The negative electrode of the cell 10, commonly referredto as the anode 22, includes an anode can 24 that contains anode activematerial 26. The anode can 24 has a top wall 28 and an annulardownwardly-depending side wall 30. Top wall 28 and side wall 30 have, incombination, an inner surface 32 and outer surface 34. Side wall 30terminates in an annular can foot 36, and defines a cavity 38 within theanode can, which contains the anode material 26.

[0057] The positive electrode, commonly referred to as the cathode 40,includes a cathode assembly 42 contained within a cathode can 44.Cathode can 44 has a bottom 46 and an annular upstanding side wall 47.Bottom 46 has a generally flat inner surface 48, a generally flat outersurface 50, and an outer perimeter 52 defined on the flat outer surface50. A plurality of air ports 54 extend through the bottom 46 of thecathode can to provide avenues for air to flow into the cathode. An airreservoir 55 spaces the cathode assembly 42 from the bottom 46 and thecorresponding air ports 54. A porous diffusion layer 57 fills the airreservoir 55, and presents an outer reaction surface 90 for the oxygen.Side wall 47 of the cathode can has an inner surface 56 and an outersurface 58. It should be appreciated that an air mover (not shown) couldbe installed to assist in air circulation.

[0058] The cathode assembly 42 includes an active layer 72 that isinterposed between a barrier layer 74 and air diffusion layer 57. Activelayer 72 facilitates the reaction between the hydroxyl in theelectrolyte and the cathodic oxygen of the air. Barrier layer 74 is amicro-porous plastic membrane, typically polypropylene, having theprimary function of preventing anodic zinc particles from coming intophysical contact with the remaining elements of the cathode assembly 42.Barrier layer 74 however, does permit passage of hydroxyl ions and watertherethrough to the cathode assembly.

[0059] The anode 22 is electrically insulated from the cathode 40 via aseal, that includes an annular side wall 62 disposed between theupstanding side wall 47 of the cathode can and the downwardly-dependingside wall 30 of the anode can. A seal foot 64 is disposed generallybetween the can foot 36 of the anode can and the cathode assembly 42. Aseal top 66 is positioned at the locus where the side wall 62 of seal 60extends from between the side walls 30 and 47 adjacent the top of thecell.

[0060] The outer surface 68 of the cell 10 is thus defined by portionsof the outer surface 34 of the top of the anode can, outer surface 58 ofthe side wall 47 of the cathode can, outer surface 50 of the bottom ofthe cathode can, and the top 66 of seal 60.

[0061] As is detailed below, the cathode 16 is loaded with manganeseoxide using any of several methods to obtain an oxide-coated carbonsupport in which the carbon is substantially unoxidized. Cathodes madewith the coated support of the invention achieve higher operatingvoltages than prior art cathodes, and furthermore to improve long-termperformance during operation.

[0062] In an ex situ method for fabricating a carbon-based air cathodewith manganese dioxide catalyst particles, a solution of an oxidizingagent (potassium permanganate) and a reducing agent (sodium formate) canbe combined at room temperature and at a generally neutral pH (rangefrom about 6 to 8) to produce a manganese oxide sol according to thefollowing reaction. The manganese oxide suspensions can also be preparedfrom the reduction of potassium permanganate solution by sodium formatein acidic or alkaline solutions.

4KMnO₄+6HCOONa→4MnO₂+3CO₂+3H₂O+2K₂CO₃+3Na₂CO₃  (1)

[0063] Next, the particles in suspension are mixed with a carbon slurrythat comprises the carbon support and the mixture is stirred to dispersethe manganese oxide particles into the carbon matrix. If desired, thesuspension of carbon slurry and manganese oxide can be heated. Asuspension of manganese oxide on carbon is thus produced in a slurryform.

[0064] Alternatively, in a method for depositing the catalyst on thecarbon substrate in situ, the carbon substrate is highly agitated.Separate streams or sprays of the oxidizing agent and the reducing agentcan be mixed above the agitated carbon substrate and react with oneanother to form small oxide particles, preferably colloidal particles,before contacting the surface of the carbon support. When the particlescontact the carbon support, they can be immediately adsorbed to andevenly dispersed on the support which can further act as a seed ornucleating surface. Because the conditions favor a redox reactionbetween the oxidizing and reducing agents over the reaction between theoxidizing agent and the carbon, the available oxidizing agent issubstantially consumed before it has an opportunity to contact thecarbon substrate such that the substrate is not oxidized in the process.Because the particles and the carbon support can have net oppositecharges, the particles can be attracted to and adsorbed on the carbonsurface.

[0065] Without regard to whether the catalyst oxide-coated carbonsupport was prepared by in situ or ex situ method, the processingcontinues in a standard manner to produce an air cathode. Briefly, thebinder/waterproofing agent is added to the suspension, and the resultingmixture is stirred prior to filtering and washing. In a preferredembodiment, 25 grams of Teflon T-30 PTFE suspension are added to thesuspension, though other waterproofing agents could be used, such aspolyethylene.

[0066] The mixture is filtered and rinsed with H₂O to remove any solubleimpurities before being dried at step 118. In particular, the mixture isdried at 90° C. for approximately 14 hours in accordance with thepreferred embodiment. Finally, it is rolled to provide an activecatalyst layer for the resulting air cathode. The catalyst layer is thenlaminated to a nickel screen current collector at its inner surface anda PTFE layer at its outer surface to provide an air diffusion layer. Inorder to prevent electrical contact between the air cathode and anode, aseparator is applied on the inner surface of the nickel screen. Theseparator can comprise a traditional non-woven fabric, or couldalternatively comprise a conformal separator, as is understood by onehaving ordinary skill in the art.

[0067] The fabrication process is completed to produce a carbon-basedair cathode loaded with substantially evenly distributed manganesedioxide particles that provide a catalyst to the oxygen reductionreaction that occurs during discharge of the cell. The cathode may thenbe installed into a metal-air cell in a conventional manner.

EXAMPLE 1

[0068] For comparison, a conventional method for preparing an aircathode was undertaken. A carbon slurry was prepared by placing 1700 mLof distilled water in a mixing vessel and adding 490 grams of PWAactivated carbon (Calgon) and stirring the mixture for 30 minutes. A0.35M solution of KMnO₄ was prepared and 773 grams of that solution wasslowly poured into the mixing vessel containing the carbon slurry. Thismixture was stirred for an additional 30 minutes at room temperature. 10g of Black Pearls 2000 conducting carbon black was added and mixed foran additional 10 minutes. A waterproofing agent, primarily T-30suspension (DuPont), was added to the aforementioned mixture in theamount of 125 grams. This final mixture was stirred for an additional 15minutes.

[0069] The resultant cathode mixture was filtered through a Buchnerfunnel, rinsed with distilled water and filtered again. The resultantmix remaining in the filter was then dried at approximately 90° C. inair for 8-24 hours. Once dried, the mix was pulverized in a highintensity mixer for approximately 10 minutes. It was then rolled toprovide the active catalytic layer and subsequently laminated onto thecurrent collector to yield the final electrode. The final drycomposition contains 3.8% MnO₂. All of the carbon was activated by thepermanganate.

EXAMPLE 2

[0070] The procedure of Example 1 was repeated, except that theconcentration of KMnO₄ solution was increased to yield 8% MnO₂ in thefinal product. All of the carbon was activated by the permanganate.

EXAMPLE 3

[0071] The procedure of Example 1 was repeated, except that only onehalf of the total amount of carbon was mixed with the KMnO₄ solution andallowed to react. Then, the remaining half of the carbon was added tothe mixture and processing continued as in Example 1. This resulted in acathode in which the carbon was 50% oxidized (activated).

EXAMPLE 4

[0072] To produce a substantially unoxidized support by an ex situmethod, the following steps were performed. A sodium formate solutionwas prepared by first placing 180 grams of distilled water in a tank,and adding 20 grams of sodium formate powder. The mixture was stirredfor approximately five minutes. Next, 350 ml of potassium permanganatesolution (1.73N) was added to the sodium formate solution, and theresulting mixture was stirred for approximately an additional 10 minutesat a temperature between 25° and 100° C. to produce a manganese oxidesuspension according to Reaction (1) above. The amorphous manganeseoxide particles produced ranged in size from about 100 nanometers toabout 30 microns, and had an average particle aggregate size in therange of 20 to 26 microns.

[0073] A carbon slurry was prepared by placing 500 grams of distilledwater in a tank, adding 7 grams of Black Pearls 2000, and 93 grams ofNorit SX1G (having a BET surface area of approximately 1500 m²/g and 900m²/g, respectively), and stirring the mixture for 15 minutes. Themanganese oxide suspension was poured into a tank containing the carbonslurry, and the suspension was stirred for approximately one hour. Awaterproofing agent, and in particular 25 grams of Teflon T-30, wasadded to the suspension and the resulting mixture was stirred forapproximately 10 minutes.

[0074] The resulting cathode mixture was then treated to provide acathode. In particular, the mixture was filtered on Buchner funnel, andrinsed with H₂O before being dried at 90° C. for approximately 14 hours.Finally, it was rolled to provide the catalyst active layer. Thecatalyst layer was then laminated and treated in the manner describedabove to produce a cathode.

EXAMPLE 5

[0075] To produce a substantially unoxidized support by an in situmethod, the method of Example 4 was repeated to produce a cathode usingthe same components, except the carbon slurry was agitated for 10minutes, and then the potassium permanganate solution and the sodiumformate solution were simultaneously added slowly at room temperature soas to enter the vigorously stirred slurry as a single stream.

EXAMPLE 6

[0076] To illustrate the effect of carbon oxidation on thehydrophobicity and high current capability, polarization curves of thecathodes of Examples 1-5 were obtained as is shown in FIG. 2.Polarization measurements were carried out in a single compartment cellwith three-electrode configuration using Solartron 1286 with CorrWarefor Windows. The electrode potential was measured and referred to zincwire reference, while the counter electrode was made of Platinum gauze.Attention is directed to the capability of the cathodes at greater than50 mA/cm2 (high current density). When all of the carbon support isoxidized (Example 1), the voltage drops off quite significantly, evenwhen the amount of MnO₂ catalyst was increased from 3.8% to 8% (Example2). When only half the carbon is oxidized but the amount of MnO₂catalyst is maintained at 3.8% (Example 3), the high current capabilityis significantly improved. Notably, however, superior performance wasobserved where the carbon was prepared in accordance with the invention(Examples 4 and 5).

EXAMPLE 7

[0077] Long-term performance tests of carbon based air cathodes wereconducted at constant current density over a period of time andrecording the corresponding changes in electrode potential. This testdetermines the sustained robustness of the electrodes, particularly fromflooding in the mass transport controlled region of the polarizationcurves of FIG. 3. A rapid drop off in voltage implies that the threephase interface in the structure and the hydrophobicity of the finishedelectrode are inadequate to sustain high currents. The electrodes werethe same as those in FIG. 2, except the electrode of Example. 5 is notshown.

[0078] A very high current density of 200 mA/cm2 was applied to stressthe electrodes. The results show a substantial performance improvementfor the present invention compared to all others, with minimal voltagedrop over the duration of the test.

EXAMPLE 8

[0079] It has further been determined that the present cathode producesa metal-air cell having an increased operating voltage, when compared toprior art metal-air cells. In particular, referring to FIG. 4, thedischarge profile of zinc-air 13 size cells having cathodes constructedin accordance with the present invention are compared to those of theprior art. As illustrated, the present cell achieves an operatingvoltage of almost 30 mV greater than prior art cells throughout theusable life of the cell. While the present cell becomes fully depleted afew hours sooner than the conventional cell, a skilled artisan wouldappreciate that the voltage of the conventional cell is substantiallylow so as to render the cell useless for its intended purpose duringthis time.

EXAMPLE 9

[0080] The cathodes of the invention were shown to exhibit the followingperformance: Current Density (I) Voltage (V) mA/cm² (V) 100 >1. 1150 >1.05 200 >0.9

[0081] The data presented demonstrate the importance of reducing oreliminating carbon oxidation when preparing a high performance aircathode.

[0082] The invention has been described in connection with what arepresently considered to be the most pratical and preferred embodiments.However, the present invention has been presented by way of illustrationand is not intended to be limited to the disclosed embodiments.Accordingly, those skilled in the art will realize that the invention isintended to encompass all modifications and alternative arrangementincluded within the spirit and scope of the invention, as set forth bythe appended claims.

We claim:
 1. A carbon support comprising at least one of conductivecarbon black and activated carbon and having an oxide catalyst loadedthereupon, the carbon support being substantially unoxidized.
 2. Acarbon support as claimed in claim 1, the conductive carbon black havinga surface area of at least 1200 m²/g.
 3. A carbon support as claimed inclaim 1, the activated carbon having a surface area of at least 700m²/g.
 4. A carbon support as claimed in claim 1, the oxide catalystcomprising manganese.
 5. A carbon support as claimed in claim 1, theoxide catalyst comprising manganese dioxide.
 6. A carbon support asclaimed in claim 1, the oxide catalyst comprising primary particles ofsubmicron size.
 7. A carbon support as claimed in claim 1, the oxidecatalyst comprising secondary particles of between about 100 nanometersand 30 microns in size.
 8. An active layer for an air cathode, theactive layer comprising a hydrophobic binder and a carbon supportcomprising at least one of conductive carbon black and activated carbonand having an oxide catalyst loaded thereupon, the carbon support beingsubstantially unoxidized.
 9. A cathode active layer as claimed in claim8 comprising, by weight, 70-80% carbon and 2-20% oxide catalyst.
 10. Anair cathode for an electrochemical cell, the air cathode comprising ametallic substrate, an active layer on a first side of the substrate, ahydrophobic diffusion layer on a second side of the substrate, theactive layer comprising a hydrophobic binder and a carbon supportcomprising at least one of conductive carbon black and activated carbonand having an oxide catalyst loaded thereupon, the carbon support beingsubstantially unoxidized.
 11. An air cathode as claimed in claim 10wherein the diffusion layer comprises PTFE.
 12. An air cathode asclaimed in claim 10 comprising, by weight, 70-80% carbon and 2-20% oxidecatalyst.
 13. An electrochemical cell comprising: an anode; an aircathode comprising a metallic substrate, an active layer on a first sideof the substrate, a hydrophobic diffusion layer on a second side of thesubstrate, the active layer comprising a hydrophobic binder and a carbonsupport comprising at least one of conductive carbon black and activatedcarbon and having an oxide catalyst loaded thereupon, the carbon supportbeing substantially unoxidized; a separator between the anode and thecathode; and an electrolyte in contact with the anode and the cathode.14. A method for making an air cathode for an electrochemical cell, themethod comprising the steps of: combining a carbon slurry comprising atleast one of conductive carbon black and activated carbon, with asuspension of oxide catalyst particles having particles ranging in sizefrom about 100 nanometers to about 30 microns and a waterproofing agentto form a mixture, the carbon in the mixture being substantiallyunoxidized; forming the mixture into an active layer for an air cathode,the carbon in the cathode being substantially unoxidized; andincorporating the active layer into an air cathode.
 15. A method formaking an air cathode as claimed in claim 14 wherein the suspension ofoxide catalyst particles is formed by a method comprising the step ofmixing an oxidizing agent that comprises manganese with a reducing agentto yield a suspension of oxide catalyst particles.
 16. A method formaking an air cathode as claimed in claim 14 wherein the oxidizing agentcomprises a soluble manganese compound having a valence state higherthan +4.
 17. A method for making an air cathode as claimed in claim 14wherein the oxidizing agent comprises a permanganate salt selected fromthe group consisting of a lithium salt, a sodium salt, a potassium salt,a silver salt, an ammonium salt, a cobalt salt, and a mixture thereof.18. A method for making an air cathode as claimed in claim 14 whereinthe reducing agent is selected from the group consisting of an organicreducing agent and an inorganic reducing agent.
 19. A method for makingan air cathode as claimed in claim 14 wherein the reducing agent isselected from the group consisting of a nitrate, a chloride, a sulfate,and a perchlorate of a manganese compound having a valence of +2 andhydrogen peroxide.
 20. A method for making an air cathode as claimed inclaim 14 wherein the reducing agent is selected from the groupconsisting of fumaric acid, citric acid, formic acid, a salt of any ofthe foregoing acids, an alcohol that can be readily oxidized, and analdehyde that can be readily oxidized.
 21. A method for making an aircathode for an electrochemical cell, the method comprising the steps of:mixing an oxidizing agent comprising manganese with a reducing agent andwith a carbon support under conditions that favor a redox reactionbetween the oxidizing agent and the reducing agent over a reactionbetween the oxidizing agent and the carbon to form oxide catalystparticles and to load the particles in situ onto the carbon support;mixing a waterproofing agent with the oxide-catalyst-loaded carbonsupport to form a mixture; forming the mixture into an active layer foran air cathode, the carbon in the cathode being substantiallyunoxidized; and incorporating the active layer into an air cathode. 22.A method for making an air cathode as claimed in claim 21 wherein theoxidizing agent comprises a soluble manganese compound having a valencestate higher than +4.
 23. A method for making an air cathode as claimedin claim 21 wherein the oxidizing agent comprises a permanganate saltselected from the group consisting of a lithium salt, a sodium salt, apotassium salt, a silver salt , an ammonium salt, a cobalt salt, and amixture thereof.
 24. A method for making an air cathode as claimed inclaim 21 wherein the reducing agent is selected from an organic reducingagent and an inorganic reducing agent.
 25. A method for making an aircathode as claimed in claim 21 wherein the reducing agent is selectedfrom the group consisting of a nitrate, a chloride, a sulfate, aperchlorate and hydrogen peroxide.
 26. A method for making an aircathode as claimed in claim 21 wherein the reducing agent is selectedfrom the group consisting of fumaric acid, citric acid, formic acid, asalt of any of the foregoing acids, an alcohol that can be readilyoxidized, and an aldehyde that can be readily oxidized.